www.battelle.org

Lydia Cumming1, Matthew Place1, Jennifer Hare2, Andrew Black2, Neeraj Gupta1, Allen Modroo3, and Rick Pardini3

(1Battelle, Columbus, Ohio; 2Micro-g LaCoste, Inc, Lafayette, Colorado; and 3Core Energy LLC, Traverse City, Michigan)

Field Testing the Applicability of Borehole Gravity for

Monitoring Geologic Storage of CO2 within Closed

Carbonate Reef Reservoirs

BACKGROUNDThe Midwest Regional Carbon Sequestration Partnership (MRCSP) is evaluating the potential to use Silurian-aged

reefs for the storage of CO2 as part of enhanced oil recovery (EOR) operations. The Niagaran Pinnacle Reef Trend

on the northern flank of the Michigan Basin is a regionally significant potential resource for hydrocarbon production

and CO2 storage. The project is being carried out across the pinnacle reefs in fields in different stages of their

production life-cycle, including a highly depleted reef that has already undergone CO2 -EOR (late-stage reef), reefs

currently undergoing CO2 -EOR (active EOR reefs), and reefs that have previously only experienced primary oil

production (new EOR reefs). The pinnacle reefs are oil-bearing dolomite and limestone structures that are overlain

by low-permeability carbonates and evaporates.

The use of borehole gravity (BHG) was tested in the late-stage reef (referred to as Dover 33) to assess the capability

of this technology for monitoring CO2 movement and storage within the reef structure. BHG surveys are conducted

by performing gravity measurements at a series of downhole stations within the well or borehole being tested.

Gravity (Δg) and depth (Δz) differences measured between successive stations constitute the interval vertical

gradient of gravity (Δg/Δz), which varies directly with the density of the rock layer bracketed by the measurements.

BHG data are converted to apparent or interval density profiles, which can be used for reservoir evaluation, well log

and core analysis/correlation, integration with surface gravity and seismic studies, or engineering and rock property

investigations. For CO2 monitoring, time-lapse BHG surveys may be useful for detecting changes in density caused

by the accumulation of CO2 within the pore spaces of the rock layer.

TECHNICAL APPROACH

The work was a collaborative effort between Battelle, MGL, and Core Energy. Battelle leads the MRCSP, which is supported by the U.S. DOE-NETL under Cooperative Agreement No. DE-FC26-0NT42589. The BHG tools were deployed by MGL survey services,

who also supplied the pre-survey modeling and post-survey data analysis/modeling. Core Energy provided key data, access and field implementation support.

Using existing well log and seismic data provided by Core Energy, LLC, Battelle and Micro-g LaCoste, Inc. (MGL)

developed a model to predict the change in density of the reef. Unlike a saline reservoir scenario that has CO2 replacing

brine in the pore space, the depleted pinnacle reef model scenario assumed the CO2 would be replacing low

pressure/density gas. Furthermore, the model assumed the CO2 would concentrate in the more porous zones connected

through high permeability pathways. The model predicted that the mass added to this closed reef structure would result

in a measurable positive density change over time.

Two BHG surveys were performed within an injection well located in Dover 33 to generate time-lapse data. A baseline

gravity measurement was performed in early 2013 while the reef was in a depleted, low-pressure condition. Repeat

measurements were made in the well in 2016 following the injection of approximately 265,000 metric tons of CO2 into the

reef and the reservoir pressures had increased from approximately 600 psi to 3,500 psi (4 MPa to 24 MPa).

The largest sources of spatial gravity variations are the free-air effect, latitude effect, and regional and local geology-

including terrain, lithology and structural variations. These time-static sources of gravity variation are cancelled by time-

differencing survey data, and the remaining time-lapse gravity signal is representative of temporal changes in formation

densities (such as those due to CO2 or other fluid injections or redistributions).

TESTING METHODS

▪ Two gravity surveys (2013 and 2016) were performed in the same well with the same configuration parameters.

▪ The same 39 gravity-measurement stations were quantified during each test.

▪ Station depths were verified to within a few cm between the tests using gamma ray.

▪ BHG measurements were made using a finer, vertical station interval near the zone of interest (reef/reservoir) with

more widely-spaced stations near the surface:

TEST EQUIPMENT▪ 2.4-inch diameter Gravilog –

deployed by MGL (2013)

• 7 microGal average station

accuracy

• Bubble leveling system

▪ 1.9-inch diameter Gravilog –

deployed by MGL (2016)

• 3 microGal average station

accuracy

• Micro-Electromechanical System

(MEMS) tilt sensors

▪ L-M 1-33 Well with sealed (closed-

ended) 2-7/8 inch tubing

▪ Tubing was filled with light-weight

(9.6 lb/gal) Na-Cl Brine

RESULTS

The time-lapse gravity signal over the whole well survey shows the change in gravity as small, variable and centered

around 0 (zero change) in the shallowest zone (Zone 3) and upper part of the intermediate zone (Zone 2). Below this, the

signal is positive and generally increasing down to the top of the reservoir (Zone 1), then positive and generally

decreasing down through the reservoir toward the base of the well. In Zone 1, the gravity difference between 2013 and

2016 clearly shows a broad, approximately 90 microGal peak signal change above the Niagaran Brown (NB) reservoir

(Stations 9 through 12). This signal is consistent with the forward models of CO2 injection in a closed structure.

Change in Gravity and Measured Density between the 2013 and 2016 BHG Surveys in Zone 1

The change in density between the

two BHG surveys provide an

indication of the distribution of CO2

within the reef. The largest BHG

survey density change is observed

in the upper, approximately 40 ft of

the Niagaran Brown formation

(Stations 7 and 8), where a density

change from 0.033 to 0.042 g/cm3

is indicated by the computed BHG

slab density. These density

changes indicate that CO2 has

filled the pore spaces in this portion

of the reef where the porosity is

generally greater, and that the

injected CO2 has been contained

within the reef (Brown Niagaran

formation).

*The estimated error bounds are indicated by dashed black lines on both panels. Formation tops were provided by Battelle: OWC= Oil water contact;

NB= Niagaran Brown (reservoir); A1C= A1 Carbonate (reservoir); A2A= A2 Anhydrite (seal). The well perforations are from 5,309 to 5,460 ft.

CONCLUSIONS

The geologic framework and pore-fluid distribution

models can be combined with the time-lapse gravity

monitoring to substantiate the movement of the injected

CO2 in the reef. The CO2 appears to have replaced the

original gas zone in the upper portion of the reef. The

red zone shows the area of greatest CO2 storage

followed by the green zone, which indicates significant

CO2 storage. The yellow and grey areas indicate

minimal CO2 storage.

The results obtained at the MRCSP adds to the

growing knowledge base regarding the use of BHG

surveys to monitor CO2 storage performance. This

research is important because BHG surveys could

provide an effective method for demonstrating storage

security, as well as provide valuable data to improve

geologic models. Time-Lapse Density Model Showing Heterogeneous

Distribution of CO2 (left) with Modeled (Red/Magenta) and

Observed (Black/Blue) Density and Gravity Curves (right)

Map View of Late-Stage Reef Used for the BHGM Surveys

Side View of Reef with Locations of Deepest Borehole Gravity Stations

Preparation of the Well for BHGM Surveying 2016 Gravilog Meter 2016

Zone 1 (Deep, Reservoir Zone)

▪ 20 to 40 ft (6 to 12 m) Station Spacing

▪ Depths between 5,176 and 5,540 ft (1,577 and 1,658 m)

▪ 4 Sweeps (well passes, or “Repeats”)

Zone 3 (Shallow)

▪ 190 to 380 ft (58 to 116 m) Station Spacing

▪ Depths between 660 to 3,253 ft (201 and 991 m)

▪ 3 Sweeps (Repeats)

Zone 2 (Intermediate, Above Reservoir)

▪ 120 to 280 ft (36 to 85 m) Station Spacing

▪ Depths between 3,253 and 5,176 ft (991 and 1,577 m)

▪ 3 Sweeps (Repeats)

Zone 4 (Near Surface)

▪ 3 Stations at Depths of 10, 34 and 240 ft (3, 10, and 73 m)

▪ 1 Sweep (Repeat)

Formation

Name

Station # Depth 2013 (ft) Depth 2106 (ft) ∆ Gravity (mGal) ∆ Density (g/cm3)

SBS 15 5176.0 5176.6 0.079 -0.014

A2C 14 5217.0 5217.2 0.064 -0.021

A2C 13 5247.0 5246.9 0.080 -0.012

A2A 12 5276.0 5275.9 0.089 0.019

A1C 11 5306.0 5306.0 0.075 -0.012

A1C 10 5320.0 5320.0 0.079 -0.009

A1C 9 5350.0 5350.0 0.086 0.006

NB 8 5370.0 5370.0 0.083 0.042

NB 7 5390.0 5390.0 0.062 0.033

NB 6 5410.0 5410.0 0.045 -0.013

NB 5 5430.0 5430.0 0.052 -0.022

NB 4 5450.0 5450.0 0.040 0.008

NB 3 5480.0 5480.0 0.034 0.006

NB 2 5510.0 5510.0 0.029 0.023

NB 1 5540.0 5540.1 0.012 -

Observed BHGM Gravity Change (2016 – 2013) (left), Computed BHGM Slab

Density Change (middle) with Error Bars, and Neutron Porosity (right)

The injection of 265,000 metric tons of CO2 into this sealed, depleted reservoir was expected to result in an increase in

the average density of the reef, and a greater increase in density in zones of the reef that CO2 accumulated. The data

collected during the time-lapse monitoring agree with these hypothesized results.